The present invention relates to a lead-containing glass for use in space, hereinafter referred to as “space glass”, and to a method of producing it as well as to its applications.
The invention also relates to specifically developed optical glasses having increased refractive indices of between nd=1.52 to 1.65 for the design of space-saving and light-weight imaging optics with lenses of different glass types for use in different objects travelling in space. The low total weight of the optics is decisive and can be realized more easily with lens optics than with mirror optics. Mirror optics should be used when imaging properties in the UV wavelength range below 350 nm are required, because of transmittance specifications. When optics are used in metal housings on different objects travelling through space, they are subjected to space radiation which depends on the orbit and the time period of the objects as well as the time point during the mission, and therefore, in addition to transmittance, they have to fulfil other requirements. In particular, they must have a UV-transmittance in a wavelength range between 300 and 800 nm which is as high as possible, a stability of transmittance over years which is as high as possible and nearly zero signs of aging of the materials, which would limit their applications due to compaction, haze or film-forming.
There are special requirements which must be fulfilled by space glass. Space radiation in the range of the low orbit of between 500 and 1000 km around the earth generally consists of electrons and protons and the resulting gamma radiation which is caused by collisions of the electrons and protons with other matter. When optical systems for monitoring space and for monitoring and measuring the surface of the earth are subjected to this radiation field during an exposure time of several years, especially gamma radiation at radiation doses of some 1000 rd (1 Gy=100 rd) due to the particle radiation impinging on the metal housings of the optics will also interact with the glass system as a secondary radiation.
The materials which are used in aeronautics technology have to fulfil the requirements of this special environment. In orbits in space around the earth the materials are subjected to weightlessness, extreme temperatures, temperature fluctuations, vacuum, micro meteorites, particle radiation from the upper atmosphere and electromagnetic high-energy radiation.
The most important gas in the interplanetary matter is neutral hydrogen which partially originates ionized as protons from inner areas of the planetary system. Additionally, free electrons with a frequency of ca. 5 cm−3 are present at the earth-sun (1.5×1011 m) distance. The radiation environment of the earth contains these charged particles together with heavy high-energy particles and photons of the entire electromagnetic spectrum.
When an optic is used outside the spacecraft, then all the aforesaid kinds of radiation may directly affect the optical system with negative consequences. When this primary radiation e.g. affects the housing of the camera of the optic, then gamma radiation with a high operating range is produced in optic materials with known formation of color centers and increasing loss of transmittance.
Due to the dry thrust interfering with the optics in the starting phase of the objects travelling during space missions, the geometry of single lenses of the optics is limited to diameters of ca. 150 mm and maximum thicknesses of ca. 50 mm. For the design of optics for space basically two material classes of optical glass with a wide variety of optical positions (refractive index/dispersion) and a strongly limited selection of Cerium-stabilized radiation protective glasses are available. In Cerium-stabilized radiation protective glasses the ion Ce3+/Ce4+ which is incorporated into the glass prevents discoloration due to radiation exposure, because it may act as both an electron scavenger (Ce4+) and electron donor (Ce3+). However Cerium-stabilized radiation protective glasses cannot be used as optical glasses due to their unfavorable transmittance properties.
Both glass species are not suitable for stable optics with UV-transmittance for space missions, because the not radiation-resistant optical glasses especially have aging properties in the UV-VIS-range between 300 and 800 nm in the radiation field of space and the imaging properties of the optics are negatively changed during the mission by a steadily increasing discoloration and thus the aims of the mission may be compromised.
The second group of materials is radiation-resistant, but the transmittance thereof is already limited by strong self-absorption of the stabilizer CeO2. This already applies to glasses with low doping rates of 1% by weight with readily polarizing cations.
Currently a high transmittance in the UV-VIS range is necessary for use, so only the not radiation-resistant material can be used as a component. In this case the optic has to be protected structurally against radiation by targeted shielding, which means additional volume in the spacecraft and additional mass for the mission. These measures would mean considerably higher costs for the space mission.